I've got a 54cm 2009 Trek 520 that I've been riding in the snow with 700x35 Marathon Winter tires. I'd like to go with a 700x40 Nokian tire in the front, but I think I'd have to remove the fender to do it.

It sounds like the 700c LHT fork has a bit more clearance. Has anyone put an LHT fork on a Trek 520?

Should be no reason why not- some variables are:

1) Threaded or threadless. I think both those forks are 1 1/8" threadless, so no problem.
2) Steerer length- threadless are cut shorter to fit, so the new fork needs to be long enough if your frame is really big. At 54 cm you won't have a problem.
3) Fork rake - 40-45mm is usual, changes the handling characteristics, but in my experience changing from a 45mm offset to 40mm offset didn't change things enough to be really noticeable on my road bike.
4) Axle to crown distance- make sure they're the same or close.